Non-invasive measurement techniques are increasingly accepted for the measurement of blood parameters (e.g., blood oxygen saturation). These techniques are particularly favorable since they do not require withdrawal of blood samples from the examined subject. For example, non-invasive optical measurement techniques are based on detection of light transmitted through (or reflected from) the a tissue/organ of the examined subject, and employ spectrophotometric measurements to determine the presence of various blood constituents based on known spectral behaviors of these constituents.
Many of the optical non-invasive techniques utilize an optical device or probe configured to attach to a finger of a subject, and an optical assembly for irradiating the finger with light and detecting its light response (e.g., measuring the intensity of light transmitted/reflected through/from the radiated finger). Conventional devices, such as pulse oximeters which are the generally accepted standard of everyday clinical practice, provide for measuring enhanced optical pulsatile signals caused by the changes in the volume of blood flowing through a fleshy medium (e.g., finger).
It is known that for blood parameters other than oxygen saturation, e.g., glucose concentration, significant difficulties are encountered due to their spectral absorption behavior in red and near infrared regions which is not as remarkable as for oxygenized hemoglobin. Hence, the main limitations on the way of expanding the non-invasive techniques to measurements different from pulse oximetry are associated with the limited selectivity of the absorption based method.
Occlusion-based techniques are disclosed in U.S. Pat. No. 6,400,972, WO 01/45553 and WO 01/96872, all assigned to the assignee of the present application, wherein over-systolic pressure is applied to the blood perfused fleshy medium with a normal blood flow (also referred to herein as pulsatile state) so as to create a state of temporary blood flow cessation at the measurement location. In these techniques measurements are taken using different wavelengths of incident radiation and/or different polarization states of detected light at timely separated sessions during a time period including a cessation time in which a blood flow cessation state (also referred to herein as occlusive state) is maintained. Due to the cessation of the blood flow, a condition of artificial blood kinetics is achieved, with the optical characteristics of the blood associated with the light response being different from those at normal (i.e., pulsatile) blood kinetics. It was found that owing to the effect of the artificial kinetics, the optical characteristics of blood changes dramatically, such that they differ from those of the fleshy medium with a normal blood flow by about 25 to 45%, and sometimes even by 60%. Hence, the accuracy (i.e., signal-to-noise ratio) of the technique based on the artificial kinetics as well as selectivity of the optical measurements can be substantially improved when compared with those based on measurements of the blood parameters at natural/normal kinetics.
In another non-invasive measurement technique, described in WO 2007/020647, also assigned to the assignee of the present application, measurements are performed by applying an external electromagnetic field to a measurement location in a subject and detecting at least two responses from which data indicative of the detected response is generated. The measurements are carried out under normal blood flow conditions for generation of first measured data indicative of a first time variation of the response for each of the at least two parameter values, and under a condition of artificial kinetics for generation of second measured data indicative of a second time variation of the response for each of said at least two parameter values. The first and second measured data are processed to determine a first relation between the first time variations for the different parameter values and a second relation between the second time variations for these different parameter values. The first and second relations are then utilized to determine the at least one blood and/or tissue related parameter.
U.S. Pat. Nos. 5,776,071, 6,190,325 6,196,974, and US Patent application No. 2002/161305, describe blood-pressure monitoring apparatuses including a measuring device using an inflatable cuff to apply a pressing pressure to a body portion of a living subject and measure at least one blood-pressure value of the living subject by changing the pressing pressure of the inflatable cuff.
U.S. Pat. No. 8,419,649 describes a method and apparatus for continuous measurement of blood pressure, based on pulse transit time, which does not require any external calibration. A body-won sensor is used to measure blood pressure and other vital signs, and wirelessly transmit them to a remote monitor. A network of disposable sensors, typically placed on the patient's right arm and chest, connect to the body sensor and measure a time-dependent electrical waveform, optical waveform, and pressure waveform. The disposable sensors typically include an armband that features an inflatable bladder coupled to a pressure sensor, at least 3 electrical sensors, and an optical sensor attached to a wrist-worn band.